fix: properly hydrate already-resolved async blocks (alternative) (#17641)

This is basically #17611, minus #17640, plus #17639. We need to add the $.next() call after render tags as well as components; rather than duplicating the logic, we can use is_standalone to determine when this is necessary (since this is what prevents $.append(...) from being used).

Fixes #17261
Fixes #17608
---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

Author: Rich-Harris

.changeset/poor-students-nail.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: properly hydrate already-resolved async blocks

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -166,6 +166,7 @@ export function client_component(analysis, options) {
 		in_constructor: false,
 		instance_level_snippets: [],
 		module_level_snippets: [],
+		is_standalone: false,
 
 		// these are set inside the `Fragment` visitor, and cannot be used until then
 		init: /** @type {any} */ (null),

packages/svelte/src/compiler/phases/3-transform/client/types.d.ts

@@ -83,6 +83,9 @@ export interface ComponentClientTransformState extends ClientTransformState {
 	readonly instance_level_snippets: VariableDeclaration[];
 	/** Snippets hoisted to the module */
 	readonly module_level_snippets: VariableDeclaration[];
+
+	/** True if the current node is a) a component or render tag and b) the sole child of a block  */
+	readonly is_standalone: boolean;
 }
 
 export type Context = import('zimmerframe').Context<AST.SvelteNode, ClientTransformState>;

packages/svelte/src/compiler/phases/3-transform/client/visitors/Fragment.js

@@ -122,7 +122,10 @@ export function Fragment(node, context) {
 			close = b.stmt(b.call('$.append', b.id('$$anchor'), id));
 		} else if (is_standalone) {
 			// no need to create a template, we can just use the existing block's anchor
-			process_children(trimmed, () => b.id('$$anchor'), false, { ...context, state });
+			process_children(trimmed, () => b.id('$$anchor'), false, {
+				...context,
+				state: { ...state, is_standalone }
+			});
 		} else {
 			/** @type {(is_text: boolean) => Expression} */
 			const expression = (is_text) => b.call('$.first_child', id, is_text && b.true);

packages/svelte/src/compiler/phases/3-transform/client/visitors/RenderTag.js

@@ -85,6 +85,10 @@ export function RenderTag(node, context) {
 				)
 			)
 		);
+
+		if (context.state.is_standalone) {
+			context.state.init.push(b.stmt(b.call('$.next')));
+		}
 	} else {
 		context.state.init.push(statements.length === 1 ? statements[0] : b.block(statements));
 	}

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js

@@ -461,7 +461,7 @@ export function build_component(node, component_name, loc, context) {
 		memoizer.check_blockers(node.metadata.expression);
 	}
 
-	const statements = [...snippet_declarations, ...memoizer.deriveds(context.state.analysis.runes)];
+	let statements = [...snippet_declarations, ...memoizer.deriveds(context.state.analysis.runes)];
 
 	if (is_component_dynamic) {
 		const prev = fn;
@@ -515,15 +515,21 @@ export function build_component(node, component_name, loc, context) {
 	const blockers = memoizer.blockers();
 
 	if (async_values || blockers) {
-		return b.stmt(
-			b.call(
-				'$.async',
-				anchor,
-				blockers,
-				async_values,
-				b.arrow([b.id('$$anchor'), ...memoizer.async_ids()], b.block(statements))
+		statements = [
+			b.stmt(
+				b.call(
+					'$.async',
+					anchor,
+					blockers,
+					async_values,
+					b.arrow([b.id('$$anchor'), ...memoizer.async_ids()], b.block(statements))
+				)
 			)
-		);
+		];
+
+		if (context.state.is_standalone) {
+			statements.push(b.stmt(b.call('$.next')));
+		}
 	}
 
 	return statements.length > 1 ? b.block(statements) : statements[0];

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/component.js

@@ -101,10 +101,16 @@ export function build_inline_component(node, expression, context) {
 			}
 
 			push_prop(b.prop('init', b.key(attribute.name), value));
-		} else if (attribute.type === 'BindDirective' && attribute.name !== 'this') {
+		} else if (attribute.type === 'BindDirective') {
 			// Bindings are a bit special: we don't want to add them to (async) deriveds but we need to check if they have blockers
 			optimiser.check_blockers(attribute.metadata.expression);
 
+			if (attribute.name === 'this') {
+				// bind:this is client-only, but we still need to check for blockers to ensure
+				// the server generates matching hydration markers if the client wraps in $.async
+				continue;
+			}
+
 			if (attribute.expression.type === 'SequenceExpression') {
 				const [get, set] = /** @type {SequenceExpression} */ (context.visit(attribute.expression))
 					.expressions;

packages/svelte/tests/runtime-runes/samples/async-attach-hydration-mismatch/Inner.svelte

@@ -0,0 +1,4 @@
+<script>
+  let { children } = $props()
+</script>
+<div>{@render children?.()}</div>

packages/svelte/tests/runtime-runes/samples/async-attach-hydration-mismatch/Outer.svelte

@@ -0,0 +1,4 @@
+<script>
+  let { children } = $props()
+</script>
+<div>{@render children?.()}</div>

packages/svelte/tests/runtime-runes/samples/async-attach-hydration-mismatch/Trigger.svelte

@@ -0,0 +1,7 @@
+<script lang='ts'>
+  import type { Attachment } from 'svelte/attachments'
+
+  export function action(): Attachment<HTMLElement> {
+    return ()=>{}
+  }
+</script>